Optical fiber attenuators are well-known in the prior art. An example taught in U.S. Pat. No. 4,557,556 issued Dec. 10, 1985, relies on offsetting the core axes of two fibers and then fusing the ends. It is also old to couple a multi-mode optical fiber to a short length of fiber which strongly attenuates certain mode groups without attenuating other mode groups of differing orders. These and other teachings of the prior art notwithstanding, there is a need presently for achieving in an optical fiber transmission path attenuation of a precise and predetermined value, which will remain constant for a substantial time despite temperature and other environmental forces. One application of such an optical fiber attenuator is where the optical power in the path must be kept within the sensitivity limits of multiplexer/repeaters present in the path. More generally, the need exists for such a splice in an in-line configuration, achievable by methods able to be practiced successfully by routine technicians.